The wireless communications industry has grown exponentially in recent years. Third generation partnership project (3GPP) and Long Term Evolution (LTE) mobile telecommunication systems provide high data rate, lower latency and improved system performances. Dual connectivity (DC) is to enhance the performance, where a given User Equipment (UE) uses radio resources provided by at least two different network points that are cooperating across either ideal or non-ideal backhaul. In addition, each Enhanced Node B (eNB) involved in dual connectivity for a UE may assume different roles, a Master eNB (MeNB) and/or a Supporting eNB (SeNB). One user plane architecture and protocol enhancement for dual connectivity is the combination of a) the whole user plane for a UE of the S1 reference point (S1-U) terminating in a Master eNB (MeNB); b) bearers may be split in MeNB and the UE, where packets of a single bearer is multiplexed onto two different radio links that are served by the two different network points; c) The PDCP protocol layer handles common user plane functions for multiplexing and de-multiplexing onto two different radio links; and d) The RLC protocol (below PDCP) runs independently for each radio link of the split bearer(s). The peer RLC entities on the UE side receive the split bearers independently.
A data radio bearer (DRB) or a bearer is a stream of data packets that are to be transferred across the wireless network without changing the order of packets and with certain specific Quality of Service (QoS) characteristics. In a 3GPP LTE access network, typically a specific bearer or DRB would be setup for such a stream of data packets, where different streams of packets can be discriminated based on TCP/IP level packet filters. A “split data radio bearer” or a “split DRB” is a bearer for which where packets are multiplexed onto two different radio links that are served by the two different network points.
In the LTE radio protocol stack, the PDCP layer is located above the RLC layer and below the IP layer in the user plane. If a PDCP entity is established for DRBs mapped to RLC UM, it is associated with two UM RLC entities, one for the uplink direction and one for the downlink direction. RLC UM is an operating mode optimized for delay-sensitive and error-tolerant real time applications, especially
VoIP, and other delay-sensitive streaming services. Currently, reordering is performed in UM RLC to rearrange the processing order of RLC data PDUs in sequential order if they are received out of sequence due to the multiple Hybrid Automatic ReQuest repeat (HARQ) processes running in parallel with “stop and wait” operation. So in-order delivery of RLC SDUs to PDCP can be guaranteed except at re-establishment of RLC layer. Re-ordering for PDCP SDUs mapped to RLC UM is not needed in current LTE protocol stack.
If a PDCP entity is established for DRBs mapped on RLC AM, it is associates with AM RLC. The AM RLC is an operating mode optimized for error-sensitive and delay-tolerant non-real-time applications. Examples of such application include most of the interactive/background type services, such as web browsing and file downloading. Streaming-type services also frequently use AM RLC if the delay requirement is not too stringent. RRC message also uses the AM RLC in order to take advantage of ARQ to ensure reliability. Currently, reordering is performed in AM RLC to rearrange the processing order of RLC data PDUs in-sequence if they are received out of sequence due to the multiple HARQ processes running in parallel with “stop and wait” operation. So in-sequence delivery of RLC SDUs to PDCP can be guaranteed except at re-establishment of lower layers, which is not the case for dual connectivity.
For dual connectivity with bearer splitting at PDCP layer, PDCP protocol Sequence Numbers (SN) could be used to ensure in-order delivery of packets. Out of order sequence would be characterized by a “SN gap” between the PDCP SDUs of two parallel RLC bearers at the reception buffer, since each base station would deliver the data with different speed and usually out of order. The problem exists even without physical layer losses or fluctuation due to channel or loading conditions. The main cause of the problem is that the X2 flow control normally makes the MeNB sends RLC PDUs in batches to SeNB. So re-ordering at the PDCP layer is required for downlink (DL) data reception at the UE side to guarantee the in-order delivery of upper layer PDUs. Re-ordering method for PDCP SDUs of DRBs mapped on RLC UM is provided when dual connectivity is configured.
Further, due to the radio link degradation of SeNB, the UE cannot receive the data packets correctly, especially when suffering radio link failure. It is also possible that there is data loss when PDCP data PDUs are forwarded over the X2 interface between MeNB and SeNB. How to deal with the genuine loss over the Uu interface needs to be considered because the UE cannot differentiate whether the appearance of the “SN gap” is due to genuine data loss or out of delivery between the two eNBs. Re-ordering methods for PDCP SDUs of DRBs mapped on RLC AM are provided when dual connectivity is configured.
Improvements and enhancements are required for reordering PDCP packets in a dual connectivity system.